The rotor of the motor of an electric drill or the like typically has a ferrous core formed of a stack of iron plates carried on a shaft and defining a cylindrical outside surface centered on the axis. This core is formed with a plurality of axially extending, radially outwardly open, and angularly equispaced slots. A winding is laid mainly into these slots but projects axially past both core ends. At one end the turns of the winding are connected to a commutator ring by means of which electricity is fed to the core and at the other ends they reverse to run back down the rotor.
When such a motor is used in a portable tool such as a drill the motor shaft carries a pinion that drives a gear train that itself drives the bit. This shaft also carries a fan that normally pulls air into the back of the motor housing to create a stream of air that passes axially through the motor over the rotor to keep it cool. The rotation rate of this type of universal motor is typically very high, above 10,000 rev/min, so that good air flow is obtained, and a very high stepdown can be used to drive the bit.
The main problem with this system is that the air drawn through the motor to cool it frequently entrains particles, those very particles freed by the tool itself. As a result of the high rotation rate not only does the air and particles entrained by the air move very fast, but the rotor itself is rotating at a very high rate so that the particles and rotor collide at very high speed. The windings that are exposed at the ends of the core are therefore struck by these particles. Unless measures are taken this will abrade the insulation off the wires of the winding and eventually lead to short circuiting of the motor.
It is therefore standard to provide a thick layer of paint over the ends of the winding, in addition to the insulating varnish that the wires are coated with anyhow. Another procedure is to pot the ends of the winding in a resin mass to protect it, or to provide a tape or string winding as protection. These methods are all reasonably effective as far as protecting the winding, but share the disadvantage that they substantially reduce the heat transfer between the rotor and the air, as the mechanical protection has a substantial insulating effect.
German patent document No. 1,797,835 describes a basket structure that closes the grooves in the rotor by means of individual groove caps. On the output side there are openings between the groove caps which merge into a completely smooth and closed end cover defining a space open only at these small openings. These openings allow the space within the cap to be complete filled with a potting compound so that the resultant assembly is thermally insulated and shares all the disadvantages of the other arrangements described above.
British Pat. No. 965,984 and French Pat. No. 1,490,764 both describe sheet metal holders for supporting the windings of the stator of an electric motor. As such, these holders have no protective effect, and furthermore they are only employed in motors which are not cooled by an axial flow of air.